An airplane has an engine that pushes its flight. What force pushes a glider to fly? Is it gravity? I think the flight of a glider is driven by gravity, but some people disagree with me. They say gravity acts vertical, it has no component in the horizontal direction. Am I wrong?

Please note that this question is not specifically asking about what (thrust-like) force acts opposite to the drag vector. The direction of a glider's flight path as viewed from the earth may not necessarily be opposite to the direction of the drag vector-- for example, the glider may be travelling horizontally over the ground at constant altitude, or may be rising straight up in ridge lift or wave lift-- and this question may be construed as asking what force is pushing the glider along the direction of the flight path as viewed from the surface of the earth.

$\begingroup$Comments are not for extended discussion; this conversation has been moved to chat.$\endgroup$
– Federico♦Mar 22 at 17:20

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$\begingroup$ASE users with voting privileges-- please reconsider your vote. This question should not have been closed. Since a glider can move along a trajectory (as seen from the ground) that is not opposite to the drag vector, this question legitimately invited consideration of forces other than forces that would act along the direction that a thrust vector would act, and thus is not a duplicate of aviation.stackexchange.com/questions/56352/… . Some interesting content was posted that would have been off-topic to the other question.$\endgroup$
– quiet flyerMar 22 at 18:45

7 Answers
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Airplanes do not fly because of their engines. Airplanes and gliders both fly because their wings turn thrust into lift and drag.

That simplistic answer, of course, just begs the question "how can you generate thrust without an engine?"

A glider is constantly trading altitude (potential energy) for airspeed (kinetic energy). Energy over time equals force, which in this case we call thrust. Airplanes do the same thing when descending. Both can also trade energy in the opposite direction, but only briefly until the wings stall.

The big difference is that an airplane can also turn fuel (chemical energy) into thrust, which allows sustained level or climbing flight. Gliders don't have that option.

$\begingroup$The glider turns lift into thrust by having its lift vector point forward. That in turn is done by flying on a sloped glide path. It is actually that lift force which propels the glider.$\endgroup$
– Peter KämpfMar 21 at 8:02

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$\begingroup$@PeterKāmpf How can lift tilt forward with a positive AOA?$\endgroup$
– StephenSMar 21 at 13:24

$\begingroup$well, neglegting "small scale" vortices (smaller than the plane), then the airflow vector is the negative of the aicraft velocity vector, they are in the same direction..$\endgroup$
– Carl BergerMar 21 at 17:09

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$\begingroup$@StephenS: Wings do not convert forces, wings deflect airflow. In case of stationary flight (unchanged velocity vector), the forces are in an equilibrium (sum of all force vectors equals zero). Gliders have no thrust, so that's zero. When gliding, the sum of the weight vector (m*g_vector) and the aerodynamic forces (lift_vector+drag) cancel each other in stationary descent. And, btw - energy is the integral of a force along a path, no time involved here.$\endgroup$
– Carl BergerMar 21 at 17:13

The force that pushes a glider is a component of its weight. More precisely, it's the projection of the vector weight W on the glide path. It's exactly of the same value of drag D, if the glider is flying at a constant airspeed, with no accelerations. In the picture, all vectors are forces except for U, V and w, that are horizontal-, total-, and vertical airspeed, respectively...

$W$ has two components, a component $W_n$ perpendicular to $V$, and a component $W_t$ parallel to $V$. $W_t$ is the force that pushes the glider, corresponding to the thrust that drives a powered plane.

Fly a glider on a horizontal flight path. It will slow down and stall.

Now fly the same glider in a vertical dive. It will accelerate downwards, pulled down by gravity.

Now think of gliding flight as a superposition of both states. Mostly horizontal flight with a bit of vertical dive on top. Shouldn't it be obvious that gravity keeps the glider moving?

But gravity only provides the vertical component of acceleration. The glider in a dive is only accelerated downward, not forward. Only when the pilot pulls on the stick and adds some lift on the wing will the glider experience a forward acceleration.

There you have it. Since the glider is moving along an inclined flight path, its lift vector is tilted forward which provides a bit of thrust. Of course, the lift vector is only needed because the glider flies in Earth's gravitational field, so more gravity or more mass would require an increase in lift, which again means more forward thrust. But gravity is only indirectly involved by setting the lift requirement. What really pulls the glider forward is its forward lift component.

But the glider is not only accelerated forward, but a bit downwards, too. Drag, after all, is also tilted and provides a bit of upward force. That bit needs to be balanced which is done by gravity. In a tilted reference system, it is indeed gravity which is tilted and pulls the glider forward along the tilted longitudinal axis. But if we remain in the standard earth-fixed system, only lift and drag are tilted, and lift pulls the glider forward. Gravity only contributes a downward fraction, the size of which depends on the glide ratio.

Now consider the glider in an upward moving packet of air in which it flies along a flight path which is pointing upwards. But still the pitch attitude of the glider is a bit nose down in order to tilt its lift vector forward. In this condition we now have an upward sloping flight path and still it is only lift which pulls the glider forward.

$\begingroup$Suppose the earth has no atmosphere, and you throw the glider horizontally. How does the glider move? Does it move vertically downwards, or move forward and downward along a parabola? If you say it moves along a parabola. So why does it move along a parabola when there is no lift? Not only that, but it also has acceleration. why?$\endgroup$
– enbin zhengMar 21 at 11:38

$\begingroup$Was thinking the same thing, how did they land on the moon? If you throw a glider or a golf ball (shape does not matter) in a vacuum horizontally, it moves down on a parabola because it accelerates vertically from gravity, while maintaining constant horizontal speed (no drag). This is why the ungainly LEM "flew" down to the moon from orbit with rocket power and thrusters to safely land. The modern day Falcon 9 uses drag and rockets to return to earth. Note, in orbiting, the horizontal velocity is enough for the object to "fall", but never reach the ground.$\endgroup$
– Robert DiGiovanniMar 21 at 13:00

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$\begingroup$@enbinzheng - If the glider is in a vacuum, then it is a glider in name but not action. A thrown rock does not glide.$\endgroup$
– Dean F.Mar 21 at 14:38

$\begingroup$@DeanF. It moves along a parabola because of gravity and initial velocity.$\endgroup$
– enbin zhengMar 21 at 15:38

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$\begingroup$@enbinzheng - In a vacuum, the vertical component of speed would increase due to the accelerating force of gravity. The horizontal component of speed would not increase at all. In a normal atmosphere this would be different due to the forces inherent with movement through the atmosphere. This is basic physics which is highly documented, demonstrable, and reproducible. This is one of the first labs done in any freshman university physics class.$\endgroup$
– Dean F.Mar 21 at 23:28

Enbin Zheng is correct, the gravity force is vertical, always vertical, and cannot account for horizontal motion at all. What does?

Let's take the bicycle first. On flat ground it has a gravitational force of 1 G, but does not move vertically. There for, it has an equal upwards 1 G force of terra firma (the ground) holding it at "ground level". Taxiing aircraft also experience this phenomena, as well as tractor trailers, which is why the lowly semi is around 4x more fuel efficient than the best freighter aircraft (trains outdo us by another 4x). No need for lift here.

Now put the bicycle on an inclined plane, it will "glide" down the hill with no peddling power. It is the repulsive force of the ground, not gravity, that accounts for the forward motion! Just draw the vectors.

On to gliders. No terra firma once airborne (and no peddling). The repulsive force to gravity is vertical drag. Every unpowered aircraft has the downward force of gravity pulling it down through the atmosphere. Every glide starts with falling. The drag vector, like the gravity repulsive vector of the bicycle, opposes gravity.

But downward velocity is required for vertical drag. We pay for our drag with the h (height) in the potential energy formula mass x gravity x height.

So now tilt the glider forward by offsetting its center of gravity and center of vertical drag. (Accomplished in aircraft design with horizontal stabilizer area). We now have horizontal motion. A glide slope. It is only then the more efficient wing can take over lifting duties and "glide" the plane. Notice the Lift vector from the wing now has a "forward" horizontal component.

Notice also that a plane has much more profile area when viewed from directly underneath than from head on. This means much more drag is produced from falling a given distance than moving forward. A little like squeezing a wet mellon seed between two fingers. Behold its flight!

A good wing and a well designed glider use this "forward push" with amazing efficiency, as do the greatest of the "motor gliders", modern airliners.

PS:I have learned to account for forward motion with weight vector when using plane reference. Pitched down, the weight vector can be decomposed to a "thrust" line component. This matches the logic of converting potential energy to kinetic energy by diving (works for the bicycle too).

Now for Wn. It has been shown that Wn is the force against the surface of a sliding block on an inclined plane and indeed has significance in determining drag (friction). As the plane increases, Wn decreases and Wt increases. As Wn decreases, friction decreases. Air drag is negligible. For an airplane at flying speed, air drag is not negligible, and the aircraft is held on its "plane" (glide slope) by Lvertical and Dvertical opposing W. Lhorizontal opposes Dhorizontal in a steady state glide.

In order for a glider to fly, it must generate lift to oppose its weight. To generate lift, a glider must move through the air. The motion of a glider through the air also generates drag. In a powered aircraft, the thrust from the engine opposes drag, but a glider has no engine to generate thrust. With the drag unopposed, a glider quickly slows down until it can no longer generate enough lift to oppose the weight, and it then falls to earth.

For paper airplanes and balsa gliders, the aircraft is given an initial velocity by throwing the aircraft. Some larger balsa gliders employ a catapult made from rubber bands and a tow line to provide velocity and some initial altitude. Hang-glider pilots often run and jump off the side of a hill or cliff to get going. Some hang-gliders and most sailplanes are towed aloft by a powered aircraft and then cut loose to begin the glide.

The powered aircraft that pulls the glider aloft gives the glider a certain amount of potential energy. The glider can trade the potential energy difference from a higher altitude to a lower altitude to produce kinetic energy, which means velocity. Gliders are always descending relative to the air in which they are flying.

Gliders are designed to be very efficient, to descend very slowly. If the pilot can locate a pocket of air that is rising faster than the glider is descending, the glider can actually gain altitude, increasing its potential energy. Pockets of rising air are called updrafts. Updrafts are found when a wind blowing at a hill or mountain has to rise to climb over it. Updrafts can also be found over dark land masses that absorb heat from the sun. The heat from the ground warms the surrounding air, which causes the air to rise. Rising pockets of hot air are called thermals. Large gliding birds, such as owls and hawks, are often seen circling inside a thermal to gain altitude without flapping their wings. Gliders do exactly the same thing.

$\begingroup$As long as there is gravity, there will be gravitational potential energy. So the thrust of the glider flight comes from gravity. Right?$\endgroup$
– enbin zhengMar 21 at 7:08

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$\begingroup$@enbinzheng yes. See xxavier's answer, it has a good picture of this.$\endgroup$
– Jpe61Mar 21 at 7:16

$\begingroup$@Jpe61 His answer seems to be wrong. What is U? Where does it come from?$\endgroup$
– enbin zhengMar 21 at 7:54

$\begingroup$@enbinzheng - U is the horizontal component of Lift. See Robert DGV’s bicycle example. Gravity is involved. But gravity can only provide force downward. Forward motion must be provided by a horizontal force. In a non-powered, parabolic flight, the forward motion has already been provided by other means such as the initial launch by hand, rocket, or engines. Without that initial thrust, the flight would be straight up then straight down unless acted upon by another force besides gravity. With an atmosphere, that force is the air itself. Google ballistic experiments and air friction.$\endgroup$
– Dean F.Mar 21 at 14:46

$\begingroup$@enbinzheng - Another example of this is flight inside uprising air such as an updraft/thermal. A pilot can adjust their both their horizontal and their vertical speed by changing the vectors of the total lift force. If you change pitch or bank, you will redirect lift. Thereby changing the intensity of the force in the horizontal. Gravity itself has not changed. At a given altitude, it is more or less a constant. The change in velocity in this case is independent of gravity. As a matter of fact, lift can be produced when there is no gravity. All you need is moving air.$\endgroup$
– Dean F.Mar 21 at 15:03

Newton's first law of motion is Force = Mass times Acceleration. In linear straight-line flight, acceleration is zero, so net force is zero. No net force exists in linear (straight-line) gliding flight.

We should then consider the exact meaning of the words "What force pushes a glider to fly?"

These words are basically asking what are all the forces that exist that exert a component parallel to the flight path, acting in the generally forward rather than the generally backward direction. (Note that the question did not use the word "thrust", which has a different, specific, well-defined meaning in aviation. "Thrust" is zero in gliding flight.)

The answer to the question depends on precisely what we mean by "flight path".

If we are looking at the flight path through the airmass, there is no such aerodynamic force, but gravity does exert a force component that acts against the direction of the drag vector, i.e. parallel to the flight path and acting in the generally forward direction, even though gravity is purely vertical. So there's one answer.

If we are looking at the flight path relative to the ground, the answer gets more complicated, and depends on whether the glider is descending, climbing, or maintaining constant altitude. Consider the case where a glider is maintaining exactly constant altitude in slope lift. Now what force is providing a component acting parallel to the trajectory, in the generally forward direction? Not gravity. And while the net aerodynamic force acts straight up vertically, the component of the net aerodynamic force that we call the lift vector does contain a component that acts parallel to the trajectory in the generally forward direction, and thus opposes the component of the drag vector that acts parallel to the trajectory in the generally backwards direction. But take care not to confuse this statement with a claim that lift is actually helping to oppose drag-- that's not the case. Lift and drag are orthogonal (i.e. are perpendicular to each other).

The argument immediately above may strike some readers as an arcane way of playing "games" with components of vectors. But in truth the same could be said of the claim that gravity is helping to "push" a glider through the air in some way. The key point is that in straight-line gliding flight, lift, drag, and weight form a closed vector triangle, with zero net force. Which components of this triangle may be considered to be contributing a component that "pushes" forward along the trajectory, depends upon which reference frame we are viewing the trajectory from.

The analysis gets even weirder if we are looking at the trajectory relative to the ground, and the glider is climbing. There are even valid reference frames where the glider is moving backwards. Now what is the direction of the force component that we would consider to be purely a "pushing" force?

For example, consider a glider that is slowly ascending straight up relative to the ground in powerful mountain wave lift. This happens often. Since the lift and drag vector are always defined relative to the airmass rather than relative to the ground, for a given steady-state airspeed they retain the same orientation in space for a given angle-of-attack of the wing, just as the glider retains the same pitch attitude in space, no matter which way the airmass is moving relative to the ground. As the glider rises slowly straight up, it's obvious that the lift vector and the drag vector now both contain components that act in the direction of the trajectory relative to the ground, while the weight vector does not. Similarly, it's not hard to imagine a case where the glider is drifting backwards and climbing along such a trajectory that only the drag vector contains any component that acts along the directory of the trajectory relative to the ground.

The idea that a component of the lift vector is helping to pull the glider forward along the aircraft's trajectory as viewed from the ground is only true when the aircraft's achieved glide ratio is better than the L/D ratio, or when the aircraft is climbing (unless the aircraft is drifting backwards along a climb path that is flatter than the direction of the drag vector). If there is no tailwind, this means that the air must be rising.

It's usually most useful to focus on the glider's flight path through the airmass rather than the glider's trajectory relative to the ground, or relative to other reference frames, but all of these points of view are technically valid.

When we are using the glider's flight path through the airmass as our reference frame, the answer to the question ""What force pushes a glider to fly?" is "the component of the weight or gravity vector which acts parallel to the flight path." In this reference frame, neither lift nor drag are exerting any "pushing" force, i.e. any force acting parallel to the flight path and pointing generally forward rather than backwards.

Note that a related question may be asked-- "what powers a glider in flight"? Work is force times distance, and power is work per time. Again, the answer will depend on whether we are looking at the work done along the direction of the trajectory relative to the airmass, or the direction of the trajectory relative the ground. In the former case the answer is simply "the component of the weight vector that acts parallel to the flight path", while in the latter case the answer depends on the direction that the airmass is moving relative to the ground.